poodr way

gistfile1.rb

class Gear
  attr_reader :chainring, :cog, :wheel
  def initialize(chainring, cog, rim, tire)
    @chainring = chainring
    @cog = cog
    @wheel = Wheel.new(rim, tire)
  end
  def ratio
    chainring / cog.to_f
  end
  def gear_inches
    ratio * wheel.diameter
  end
  Wheel = Struct.new(:rim, :tire) do 
    def diameter
      rim + (tire * 2) 
    end
  end 
end